1. Field of the Invention
The present invention relates to drum brake shoes and brake lining blocks for vehicles such as trucks or buses.
2. Background Art
Vehicle brakes are generally either disk brakes or drum brakes. Drum brakes are generally preferred for buses and commercial trucks because drum brakes may provide a greater surface area of friction material. The braking surface of the friction material is generally riveted to a backing plate of a brake block. Generally, 12 to 16 rivets are required to attach a brake block to a brake shoe due to the substantial shear forces that must be withstood during braking between the brake blocks and the brake shoe.
Commercial and industrial vehicle brake linings that are riveted to steel brake shoes are generally rigid, non-asbestos friction material. If the outer diameter of the brake shoe surface that is to be retrofit with new brake linings is not completely flat or within original design radius tolerances, the lining after securing to the shoe may crack or break off of the brake shoe. This may create problems relating to braking performance and durability.
The paramount consideration for brake design is safety. Worn or damaged brakes can cause reduced braking performance that may result in longer stopping distances. If a brake lining becomes dislodged in whole or in part, the brake for a wheel can jam or lock-up and could result in a truck jack knifing.
Rivet holes in friction material interrupt the smooth surface of the friction material and reduce the available braking surface. Reduction of the braking surface can increase temperature stress on the friction material. Corners of the friction material can be damaged during assembly or in use and heat stress at the corners can lead to fractures.
The shear forces developed between the friction material and a backing plate or brake shoe must be withstood by the bond between the friction material and the surface to which it is secured. Shear forces between the surface of a backing plate and a brake shoe must be withstood by the fasteners that secure the brake plate that includes a backing plate to the brake shoe. If a brake plate or part of the friction lining material becomes detached from the brake shoe in an emergency, the brake assembly could lock-up or be otherwise seriously compromised.
The friction material used on brake blocks is drilled and countersunk to receive rivets that are used to connect the brake block to the brake shoe. Holes are formed in the friction material by either a drilling or a punch operation. If the counter bore is not drilled deep enough, the brake block may crack when riveted to the to the brake shoe by a riveter. If the counter bore is drilled too deep, the brake block may be only loosely fastened. This condition can lead to cracking of the friction material or other problems.
If the rivet tool misses a rivet location and contacts the friction material, it may harm the friction material. The friction material may be cracked, broken, or deformed if the rivet is misdirected, or improperly set-up for depth and rivet application force. Any cracking, breakage or deformation could adversely impact braking performance of the lined brake shoe when installed on a vehicle.
Substantial labor costs are incurred to rivet brake blocks to a brake shoe. Additional expenses may be incurred if it is necessary to scrap a cracked brake block.
The surface area of the friction material is reduced by each access hole in the friction material required for a rivet. Any reduction in surface area of the friction material may adversely affect stopping performance. Holes for rivets in the surface of the friction material may result in unwanted noise especially if dirt or other foreign material is permitted to collect in the rivet access holes.
In the manufacture of brake blocks, problems may be encountered including blistering at the corners of the brake blocks. The corners of brake blocks may be broken or damaged especially if the corners are formed with right angle corners.
There is a need for an improved brake shoe having brake blocks that maximize the friction material surface area while minimizing riveting operations. There is also a need to eliminate the potential for damage to friction material on brake blocks when they are assembled to a brake shoe.
There is also a need in some applications to eliminate riveting operations in the assembly of brake blocks to brake shoes. Some repair facilities prefer to avoid such riveting operations and instead install brake blocks using only conventional threaded fastening tools.
Conventional brake blocks for conventional drum brakes have a partial cylindrical backing plate that is mated to the partial cylindrical surface of the brake shoe. Rivets secure the brake blocks to the brake shoe and are the sole mechanism for resisting sheer forces between the brake blocks and the brake shoe when the brake engages the brake drum. Sheer forces develop as the brake shoe is pressed against the drum because brake drum rotation is slowed by contact with the friction material of the brake blocks.
When a brake is used in reverse, the rotational forces applied to the brake blocks are reversed. If the brakes are frequently used in reverse or if the brakes are applied in an emergency stop while the vehicle is moving in reverse, the rivets may distort or stretch rivet holes in the friction material. If the rivets holes become enlarged the friction material may become loose. Any looseness of the rivets relative to the friction material may create unwanted noise or improper brake feel.
With current drum brake shoes, when the lining is worn to a point beyond the recommended extent, as much as 95% of the steel shoe may contact the brake drum. When brake shoes are rebuilt and reinstalled in a vehicle, the first brake application may result in only 40% to 50% of the surface of the friction material actually contacting the brake drum. In a short period of time, of over 3 or 4 days or possibly 50 to 100 brake applications, the friction material may wear until 75%-80% of the friction material surface contacts the brake drum. Braking performance is improved as the proportion of friction material surface contacting the brake drum increases.
There is a need for an improved brake shoe having brake blocks that have improved resistance to sheer forces, reduced manufacturing costs, reduced friction material waste, reduced break-in period, and that provide maximum friction material surface contact even when the friction material is worn to the maximum extent. There is also a need for brake shoes having brake blocks that are designed to minimize noise and vibration.
Applicant's invention is directed to solving the above problems as summarized below.